In uninterrupted power supply (UPS) devices and systems two installation types are commonly used, one installation type for three-wire AC power supplies and another installation type for four-wire AC power supplies. The two installation types both refer to a three-phase alternate current and differ in the presence of a Neutral N wire provided by the AC power supply in case of the four-wire AC power supply.
Such a typical UPS device 10 according to the state of the Art can be seen in FIG. 1. The UPS device 10 comprises an AC power supply input 12, which is connected to an AC power supply 14. The AC power supply 14 is a 4-phase AC power supply 114 including Neutral N. The AC power supply 14 is connected to the AC power supply input 12 via a disconnect device 16, which is provided to disconnect all four phases including Neutral N. The UPS device 10 further comprises a DC power supply input 18 for connection of a DC power supply 20 and a power output 22 for connection of a load 24.
For providing power to the load 24, the UPS device 10 comprises an AC/DC converter 26, also referred to as rectifier, which is connected to the AC power supply input 12 to receive AC power from the AC power supply 14, a DC/DC converter 28, which is connected to the DC power supply input 18 to receive DC power from the DC power supply 20, and a DC/AC converter 30, also referred to as inverter, which is connected to the power output 22 to provide AC power to the load 24. The AC/DC converter 26, the DC/DC converter 28, and the DC/AC converter 30 are interconnected by a DC link 32. The DC link 32 comprises a storage capacitor 34 for buffering. The AC/DC converter 26, the DC/DC converter 28, and the DC/AC converter 30 with the DC link 32 form a power conversion unit 38. Instead of the DC/AC converter 30, a DC/DC converter can be used to power a DC load.
In case of failures on a side of the load 24, e.g. a short circuit, it can be required to provide a current to the load 24 which is sufficient to trigger load side protection means, since the current through the DC/AC converter 30 is limited based on the semiconductor components. This is typically achieved by a bypass connection 39, which is provided with a bypass switch 40. The bypass connection 39 is connected to an alternate AC power supply 41, which provides a current to the load 24 in case of failure to trigger protection means at a load side of the UPS device 10 and to clear the fault. In this embodiment, an additional switch 42 is provided to connect and disconnect the alternate AC power supply 41.
The UPS device 10 further comprises a control unit 44, which controls all components of the UPS device 10. The control unit 44 has an internal control reference for all voltage measurements. The internal control reference typically is a potential that is stable under normal operating conditions. One typical voltage frequently used as internal control reference is e.g. the internal Neutral N.
Furthermore, the UPS device 10 comprises a user interface and a communication interface 46, which are depicted together as one component in FIG. 1. However, in alternative embodiments, the user interface and the communication interface 46 can be individual components. The communication interface is used e.g. for communication of different UPS devices when operated together as a UPS system, which is not shown in FIG. 1.
The installations for UPS devices and systems are usually according to a pre-determined configuration or installation class, e.g. grounded (TN-S/-C) or floating (IT). The UPS device 10 shown in FIG. 1 comprises an installation as TN-S, grounded 4-wire with Neutral and Ground routed to equipment as separate conductors. This corresponds to standard European installation practice. Change of the installation from one class to another during operation is usually not recommended and under some conditions even not allowed, since such a change may introduce a direct safety risk and/or place unintentional and excessive stress on components.
However, for instance for the very common 4-wire TN-S installation, it is possible and sometimes even a regional requirement to have an external disconnect device at the AC power system input, which switches all poles of the AC power supply including neutral. The operation of the disconnect device will turn the UPS device to power the load from the DC power source connected to the DC power supply input. Additionally, the installation of the UPS device turns to IT because of the removal of installation reference from load side.
The above installation problem could be overcome by a fixed or hardwired load reference connection, e.g. to Neutral or ground, under varying modes of operation. However, the load reference connection is optional and depends e.g. on application specific and/or regional requirements. Accordingly, there is no universal solution to fix the above installation problem.
A special area of application is UPS devices and involving generators or other separately derived sources that may introduce a detrimental change of control or safety reference during or after a change of power source or the mode of operation.
The above principles have been discussed in respect to UPS devices. However, the same principles apply to UPS systems, which comprise multiple UPS devices.
UPS are for example known from US 2014/333138 A1, US 2014/104896 A1 or DE 10 2010 000 502 A1.